Sun and moon will perform a rare pas de deux in Humboldt skies on Sunday

Everything you need to know about solar eclipses can be summed up in one number: 400. During an eclipse of the sun, when the moon comes between our star and the Earth, it's a near-perfect fit: they look just about the same size. That's because, while the diameter of the sun is some 400 times greater than that of the moon, the sun is about 400 times farther away. Pure fluke. When the moon is close to us in its elliptical orbit, the lunar disk appears slightly larger than that of the sun, so when they're in line, we experience a total eclipse of the sun. This Sunday, however, the moon is nearly at "apogee" -- when it's farthest from us -- so its disk is smaller than that of the sun, giving us an annular, or ring, eclipse.

In any given spot, an annular eclipse -- with a full ring of light visible around the moon -- is a rare thing, about as common as a total eclipse of the sun. Humboldt won't see another for decades. From 2001 on through 2020, just 15 annular eclipses will be visible from anywhere on the planet, along with 13 total eclipses and a couple of hybrids (total or annular, depending on your location).

Visually, an annular eclipse is a starkly different experience from totality, when no sunlight gets past the moon, giving the weird impression of a black hole in the sky. At that time, naked eye viewing is fantastic, letting us see the corona of hot gases which surround the sun and, sometimes, pink solar flares or prominences. This Sunday, sorry: no flares, no corona. Just a ring of bright sun that's dangerous to look at directly without special eye protection. It'll feel more like a bright overcast than heavy twilight.

Even so, plenty of people are getting hyped up about the phenomenon. Locally, the centerline passes over Requa on the north bank of the Klamath, and the Requa Inn has been fully booked for nearly a year, with visitors coming from as far away as Europe. "We got our first booking for May 20 three years ago," said Jan Wortman, one of the Inn's owners, "and we've had to turn many people away. For us, this is unheard of to be booked up so far in advance."

Closer to home, Arcata's creamery district will celebrate with a free Solar Eclipse Festival, complete with stilt walkers, kinetic test drives and entertainment from the Arcata Interfaith Gospel Choir, Samba da Alegria, the Lonesome Roses and many more. Sponsors, including the Arcata Playhouse, Holly Yashi Jewelry, KHUM and the Creamery District Neighborhood Coalition, will close off a stretch of Ninth Street near L for the party, which runs from 3 to 7 p.m. Viewing should be great at eclipse time, when the sun will seem to hover right at the end of the street, said David Ferney, the playhouse's co-artistic director. Be sure to wear the "cool eclipse viewing glasses" on sale during the festival, or bring your own.

And if Arcata is foggy, you can still head for the hills, where specially filtered binoculars and telescopes will be set up for public viewing by the nonprofit Astronomers of Humboldt. The group also plans to have some low tech pinhole boxes and possibly safety glasses available. The astronomers will take over the south parking lot of the Kneeland Airport starting around 4:30 p.m., and they are encouraging people to bring picnic dinners. If the sky is clear, people can stay on for dark sky observations well into the evening, said club president Russ Owsley.

Whatever you do to mark this celestial event, be careful out there! If you're in the "path of annularity" (seeing the complete ring), about 88 percent of the sun's area will be covered. That still lets 12 percent of the sun's light through, making the ring about 50,000 times brighter than a full moon. That represents an awful lot of photons, including high-energy ultraviolet ones, plenty enough to fry a whole bunch of an unprotected retina's 100 million optical receptors in a few seconds. As with any partial eclipse of the sun, naked-eye viewing, even during the full extent of "annularity" is dangerous. (See safe viewing tips.)

In addition to the ring of sunlight, a couple of other visuals are worth looking for during the annular eclipse. One is the planet Venus, which might be visible if you look about 23 degrees east of the sun (about the span between your extended thumb and little finger held at arm's length). The other is the "pinhole camera" effect, where you'll see images of the sun's ring on the ground through small holes formed by your fingers or a straw hat, say.

Humboldtians are well placed to see the full extent of the annularity anywhere from Garberville on to the north. The sun will be low in the sky, around one-quarter of the way between the horizon and directly overhead. That's assuming the western sky will be clear, of course; historically, we have about a 50 percent chance of clear skies on the coast for the afternoon of May 20.

In Eureka, the partial eclipse begins at about 5:10 p.m., with full annularity lasting four minutes from 6:26 to 6:30 p.m. The midpoint is at 6:27:50 p.m., when the sun will be about 21 degrees above the horizon. The longest annularity for this eclipse is 5 minutes 46 seconds, nearly two minutes longer than we experience in Humboldt, occurring almost on the International Dateline in the middle of the Pacific Ocean.

Due to the International Dateline, our May 20 eclipse actually starts on May 21, in the Gulf of Tonkin, off the south China coast. It takes about two hours for the moon's shadow to swing across the Pacific, passing south of the Aleutian Islands, making landfall close to the mouth of the Klamath, and ending at sunset near Lubbock, Texas. In all, the moon's shadow travels about 8,500 miles in 3½ hours, at an average velocity of 2,400 mph. The path of annularity will be between 147 and 201 miles wide.

Babylonian astronomers, 2,500 years ago, knew that the sun, Earth and moon return to about the same relative positions in the sky roughly once every 18 years, resulting in a similar eclipse. But there is a catch. This 18-year cycle -- which has become known as the "saros" cycle -- is made up of 223 synodic (full moon to full moon) months. That works out at about 18 years 11 days and eight hours. The eight hours has the effect of shifting each eclipse one-third of an Earth-revolution west from the previous one.

We're next due partial solar eclipses in Humboldt County on Oct. 23, 2014; Aug. 21, 2017; and Oct. 14, 2023. We have to wait until Aug. 12, 2045 to experience a total eclipse of the sun here, which is actually pretty cool: Any one place on Earth only experiences totality about once every 360 years, on average. A year later, Feb. 5, 2046, Humboldt gets another annular eclipse.

If Sunday's annular eclipse doesn't quite do it for you (and truth is, the difference between a partial and a total solar eclipse is, well, day and night), be patient. The United States is due a total eclipse in five years. On Aug. 21, 2017, the centerline passes right across Salem, Ore. Trust me, it'll be worth the drive.